This invention relates to an exhaust gas recirculation control system for an internal combustion engine, and in particular to an exhaust gas recirculation feedback control system for an engine comprising an oxygen sensor for sensing the oxygen concentration in the inlet air for the engine in which a recirculated exhaust gas is mixed whereby the mixing rate (exhaust gas recirculation rate) of the recirculated gas is accurately controlled according to the output signal of the oxygen sensor.
It is well known in the art to perform so-called "exhaust gas recirculation" (hereinafter abbreviated as EGR) where a part of the exhaust gas is introduced onto the inlet side of an engine in order to reduce nitrogen oxides (hereinafter abbreviated as NOx) which are harmful components contained in the exhaust gas of an internal combustion engine. Since the flow rate of the recirculated exhaust gas affects the performance and fuel consumption, etc., of the engine in addition to the reduction rate of NOx, it is desired that the exhaust gas be accurately controlled according to the operating condition of the engine.
However, when an EGR control valve for controlling the flow rate of the exhaust gas is used for a long time, a large amount of a liquid material such as carbon contained in the exhaust gas becomes attached to the recirculation control valve so that the flow rate of the exhaust gas at the initial stage corresponding to the opening of the recirculation controlling valve is changed, resulting in inaccurate control.
U.S. Pat. No. 4,168,683 issued to Hata et al. on Sept. 25, 1979 discloses an EGR control system for an internal combustion engine in which a feedback control signal representing the amount of actually recirculated exhaust gas is produced by detecting the concentration of either CO.sub.2 or H.sub.2 O in the recirculated exhaust gas.